1. Field of the Invention
This invention relates to a lithium ion secondary battery in which positive and negative electrodes face each other sandwiching a separator which keeps an electrolytic solution, more particularly to a battery structure in which electric connection between a positive electrode and a negative electrode (electrodes) and separator is improved so that it can be made into thin form and the like optional forms and to a production method for the formation of said structure.
2. Description of the Related Art
There is a growing demand for the miniaturization and lightening of portable electronic instruments, and it is essential to improve performance of batteries to meet such a demand. Because of this, development and improvement of various batteries have been attempted in recent years with the aim of improving the battery performance. Expected characteristics of batteries to be improved include high voltage, large energy density, tolerance for large load resistance, optional shaping, safety and the like. Particularly, lithium ion battery is a secondary battery which can realize the highest voltage, largest energy density and tolerance for largest load resistance among existing batteries, and its improvement is still being made actively.
As its main composing elements, the lithium ion secondary battery has a positive electrode, a negative electrode and an ion conducting layer inserted between these electrodes. In the lithium ion secondary batteries which have been put into practical use, a plate-shaped material prepared by mixing powder of a lithium-cobalt oxide or the like active material with powder of an electron conducting substance and a binder resin and coating the mixture on an aluminum collector is used as the positive electrode, and another plate-shaped material prepared by mixing powder of a carbonaceous active material with a binder resin and coating the mixture on a copper collector is used as the negative electrode. Also, a porous film such as of polyethylene, polypropylene or the like filled with a lithium ion-containing non-aqueous solution is used as the ion conducting layer.
For example, FIG. 7 is a sectional view showing the structure of a prior art cylindrical lithium ion secondary battery disclosed in JP-A-8-83608 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). In FIG. 7, 1 is an armor case made of stainless steel or the like which also serves as a negative electrode terminal, 2 is an electrode body contained in the armor case 1, and the electrode body 2 has a structure in which a positive electrode 3, a separator 4 and a negative electrode 5 are coiled in a spiral shape. In order to maintain electric connection among the positive electrode 3, separator 4 and negative electrode 5, it is necessary to apply external pressure to surfaces of the electrode body 2. Because of this, contact among all surfaces is maintained by putting the electrode body 2 inserted into a strong metal case. In the case of a square battery, strips of electrode body are tied up into a bundle and put into a square metal case, thereby pressing them with external force.
As described in the foregoing, in the currently available lithium ion secondary batteries, strong armor cases made of metals and the like are used as a means to closely adhere positive and negative electrodes. Without the armor case, the electrodes are peeled off, so that it becomes difficult to maintain electric connection between the electrodes via an ion conducting layer (separator) and the battery characteristics therefore are deteriorated. On the other hand, not only the energy density of the battery itself is reduced because of the large weight and volume of the armor case occupying entire portion of the battery, but also shapes of the battery are limited due to rigidity of the armor case itself, thus causing a difficulty in making optional shapes.
In view of such backgrounds, development of a lithium ion secondary battery which does not require a strong armor case has been attempted with the aim of achieving lightening and thinning the battery. The point of the development of such a armor case-free battery is how to maintain electric connection of a positive electrode, a negative electrode and an ion conducting layer (separator) which is sandwiched by them, without applying external force. As such a connecting means which does not require external force, a method has been proposed in which the electrodes are closely adhered to the separator making use of a resin or the like.
For example, JP-A-5-159802 discloses a production method in which an ion conductive solid electrolyte layer and positive and negative electrodes are integrated into one body by their heat treatment using a thermoplastic resin binder. In this case, the electrodes are closely adhered to each other by integrating the electrodes and the electrolyte layer into one body, so that electric connection between the electrodes is maintained and the integrated body functions as a battery without applying external force.
Since the prior art lithium ion secondary batteries are constructed in the aforementioned manner, a battery which uses a strong armor case to ensure adhesiveness between electrodes and a separator and electric connection between electrodes is disadvantageous in producing a battery having large energy density, because the ratio of volume and weight of the non-electricity generating part armor case to the entire battery portion becomes large. Also, though a method in which electrodes are closely adhered to an ion conducting body via an adhesive resin has been proposed, it causes a problem in that ionic conduction resistance inside the battery cell increases and the battery characteristics are reduced due to large resistance of the adhesive resin layer, when a solid electrolyte layer is closely adhered to the electrodes simply via the adhesive resin.
In addition, in the case of the battery of JP-A-5-159802, the electrodes are bonded to a solid electrolytic layer with a binder, but since interfaces of the electrodes and the electrolytic layer are covered with the binder, it is disadvantageous in terms of ionic conductivity when compared for example with a case in which liquid electrolytes are used. Even if a binder having ionic conductivity is used, a material having an ionic conductivity equal to or larger than that of liquid electrolytes is not generally known, so that it causes a problem in that battery performance similar to that of a battery in which liquid electrolytes are used cannot be obtained easily.
The present invention has been accomplished as a result of intensive studies on the suitable bonding method of a separator and electrodes, conducted by the present inventors with the aim of resolving the aforementioned problems, and it contemplates providing a lithium ion secondary battery having excellent charge and discharge characteristics, which can closely adhere the electrodes to the separator without increasing ionic conduction resistance between the electrodes and without using a strong armor case, so that it has large energy density and can be made into a thin and optional shape.
A first aspect of the lithium ion secondary battery of the present invention is a battery which comprises a positive electrode having a positive electrode collector and a positive electrode active material layer formed on the positive electrode collector; a negative electrode having a negative electrode collector and a negative electrode active material layer formed on the negative electrode collector, a separator which is arranged between the positive electrode and negative keeps a lithium ion-containing electrolytic solution and a porous adhesive resin layer which bonds the positive electrode active material layer and the negative electrode active material layer to the separator and keeps the electrolytic solution to mutually connect the positive electrode, separator and negative electrode electrically.
A second aspect of the lithium ion secondary battery of the present invention is the battery according to the first aspect wherein hole ratio of the porous adhesive resin layer is equal to or larger than that of the separator.
A third aspect of the lithium ion secondary battery of the present invention is the battery according to the first aspect wherein hole ratio of the porous adhesive resin layer is more than 35%.
A fourth aspect of the lithium ion secondary battery of the present invention is the battery according to the first aspect wherein the ionic conduction resistivity of the adhesive resin layer in which the electrolytic solution is kept is set to a value equal to or lower than the ionic conduction resistivity of the separator in which the electrolytic solution is kept.
A fifth aspect of the lithium ion secondary battery of the present invention is the battery according to the first aspect wherein the bonding strength between the positive electrode active material layer and the separator is set to a value equal to or larger than the bonding strength between the positive electrode active material layer and the positive electrode collector, and the bonding strength between the negative electrode active material layer and the separator is set to a value equal to or larger than the bonding strength between the negative electrode active material layer and the negative electrode collector.
A sixth aspect of the lithium ion secondary battery of the present invention is the battery according to the first aspect wherein the adhesive resin layer is made of a fluoride resin or a mixture which uses a fluoride resin as the main component.
A seventh aspect of the lithium ion secondary battery of the present invention is the battery according to the sixth aspect wherein polyvinylidene fluoride is used as the fluoride resin.
An eighth aspect of the method of the present invention for fabricating a lithium ion secondary battery, is the method which comprises the steps of: preparing a positive electrode by forming a positive electrode active material layer on a positive electrode collector; preparing a negative electrode by forming a negative electrode active material layer on a negative electrode collector; preparing an adhesive resin solution, by dispersing a fluoride resin or a mixture containing a fluoride resin as the main component in N-methylpyrrolidone; coating the adhesive resin solution to at least one of the surface of the positive electrode active material layer and the facing surface of the separator and to at least one of the surface of the negative electrode active material layer and the facing surface of the separator; fitting the positive electrode active material layer and the negative electrode active material layer upon respective surfaces of the separator; evaporating the N-methylpyrrolidone from the adhesive resin solution to form porous adhesive resin layers so as to bond the positive electrode active material layer and the negative electrode active material layer upon respective surfaces of the separator to form a laminated body; and supplying a lithium ion-containing electrolytic solution to the laminated body.
A ninth aspect of the method of fabricating a lithium ion secondary battery according to the eighth aspect is the method wherein the adhesive resin solution contains 3-10 parts by weight of a fluoride resin or a mixture containing a fluoride resin as the main component in N-methylpyrrolidone.
A tenth aspect of the method of fabricating a lithium ion secondary battery according to the eighth aspect wherein the step of evaporating comprises a step of heating.
A eleventh aspect of the method of fabricating a lithium ion secondary battery according to the eighth aspect is the method wherein the step of coating comprises a step of coating by using a bar coator.
A twelfth aspect of the method of fabricating a lithium ion secondary battery according to the eighth aspect is the method wherein the step of coating comprises a step of coating by using a spray gun.
A thirteenth aspect of the method of fabricating a lithium ion secondary battery according to the eighth aspect is the method wherein the step of coating comprises a step of dipping the separator in an emulsified solution of the adhesive resin and then pulling it up.